Heterojunction bipolar transistors with one or more sealed airgap

ABSTRACT

The present disclosure relates to semiconductor structures and, more particularly, to a heterojunction bipolar transistor having one or more sealed airgap and methods of manufacture. The structure includes: a subcollector region in a substrate; a collector region above the substrate; a sealed airgap formed at least partly in the collector region; a base region adjacent to the collector region; and an emitter region adjacent to the base region.

FIELD OF THE INVENTION

The present disclosure relates to semiconductor structures and, moreparticularly, to a heterojunction bipolar transistor having one or moresealed airgap and methods of manufacture.

BACKGROUND

A heterojunction bipolar transistor (HBT) is a type of bipolar junctiontransistor (BJT) which uses differing semiconductor materials for theemitter and base regions or collector and base regions, creating aheterojunction. Si/SiGe HBTs are used in power amplifier applicationsand require low collector-base capacitance Ccb, high cut-off frequenciesfT/fMAX and high breakdown voltages (BVceo/BVcbo).

It is known that high Ccb (i.e., the collector-base capacitance) limitsthe ft and fmax of the HBT. In addition, traps at a corner of theshallow trench isolation region and low-doped semiconductor can resultin low bias collector leakage current. Heavy doping at the corner of theshallow trench isolation region and semiconductor (collector or baseregion) reduces the collector leakage current, but also results inhigher Ccb.

SUMMARY

In an aspect of the disclosure, a structure comprises: a subcollectorregion in a substrate; a collector region above the substrate; a sealedairgap formed at least partly in the collector region; a base regionadjacent to the collector region; and an emitter region adjacent to thebase region.

In an aspect of the disclosure, a structure comprises: a bipolar devicecomprising an emitter, base, collector and subcollector; shallow trenchisolation regions isolating the collector; and at least one sealedairgap at least partly within the collector and at a corner of theshallow trench isolation regions.

In an aspect of the disclosure, a method comprises: forming asubcollector region in a substrate; a collector region above thesubstrate; forming a sealed airgap formed at least partly in thecollector region; a base region adjacent to the collector region; andforming an emitter region adjacent to the base region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the presentdisclosure.

FIG. 1 shows a sub-collector implant, amongst other features, andrespective fabrication processes in accordance with aspects of thepresent disclosure.

FIG. 2 shows an oxide material with an opening over the sub-collectorregion, amongst other features, and respective fabrication processes inaccordance with aspects of the present disclosure.

FIG. 3 shows an epitaxial material deposited on exposed semiconductormaterial in a collector region, amongst other features, and respectivefabrication processes in accordance with aspects of the presentdisclosure.

FIG. 4 shows cavity structures at a corner of the shallow trenchisolation regions and within the collector region, amongst otherfeatures, and respective fabrication processes in accordance withaspects of the present disclosure.

FIG. 5 shows sealed airgaps, amongst other features, and respectivefabrication processes in accordance with aspects of the presentdisclosure.

FIG. 6 shows an emitter region and extrinsic base region, amongst otherfeatures, and respective fabrication processes in accordance withaspects of the present disclosure.

FIG. 7 shows contacts to the emitter region, extrinsic base region, andcollector region, amongst other features, and respective fabricationprocesses in accordance with aspects of the present disclosure.

FIG. 8 shows a heterojunction bipolar transistor with airgaps sealed byemitter material and respective fabrication processes in accordance withalternative aspects of the present disclosure.

FIG. 9 shows a heterojunction bipolar transistor with airgaps sealed byepitaxial material and surrounded by collector material and respectivefabrication processes in accordance with alternative aspects of thepresent disclosure.

FIG. 10 shows a layout of trenches (holes) used for airgap formation andrespective fabrication processes in accordance with alternative aspectsof the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to semiconductor structures and, moreparticularly, to a heterojunction bipolar transistor having one or moresealed airgap and methods of manufacture. More specifically, in onespecific embodiment, the heterojunction bipolar transistor is a SiGeheterojunction bipolar transistor with sealed airgap(s) at the interfaceof the collector region and a shallow trench isolation (STI).Advantageously, the transistors described herein provide improvedresistor performance including lower Ccb, reduced collector leakagecurrent and improved ft/fmax (compared to HBTs without airgaps).

In more specific embodiments, the transistor, e.g., SiGe HBT, includes asealed airgap (or cavity) at a triple interface, i.e., the interface ofthe extrinsic base, corner of the shallow trench isolation and thecollector region. Accordingly, the sealed airgap can be surrounded bythe extrinsic base, the shallow trench isolation, and the collectorregion. In embodiments, the airgap is sealed with an epitaxial material,e.g., SiGe. For example, cavity structures can be in-situ epitaxiallysealed (surrounded) by material of the extrinsic base, i.e., SiGe, toform the sealed airgap. In alternative embodiments, the epitaxialmaterial of the emitter region can be used to form the seal airgap. Infurther embodiments, the airgap can extend vertically through part ofthe collector region or a combination of vertically through theintrinsic base and the collector region. In further embodiments, theairgap can be a continuous layer along the length of the emitter regionor isolated holes.

It is also contemplated that the transistor can be a SiGe HBT with anepitaxially sealed airgap adjacent to the shallow trench isolation inthe semiconductor material of the collector region, i.e., silicon. Inthis embodiment, a SiGe material of the collector region is used to seala cavity structure, thereby forming a sealed airgap. The sealed airgapis thus surrounded by the collector region (e.g., silicon) on all sidesexcept for the SiGe material used to seal the airgap. The sealed airgapcan include a self planarizing Si on a top surface thereof.

The transistors of the present disclosure can be manufactured in anumber of ways using a number of different tools. In general, though,the methodologies and tools are used to form structures with dimensionsin the micrometer and nanometer scale. The methodologies, i.e.,technologies, employed to manufacture the transistors of the presentdisclosure have been adopted from integrated circuit (IC) technology.For example, the structures are built on wafers and are realized infilms of material patterned by photolithographic processes on the top ofa wafer. In particular, the fabrication of the transistors uses threebasic building blocks: (i) deposition of thin films of material on asubstrate, (ii) applying a patterned mask on top of the films byphotolithographic imaging, and (iii) etching the films selectively tothe mask.

FIG. 1 shows a subcollector implant, amongst other features, andrespective fabrication processes in accordance with aspects of thepresent disclosure. More specifically, the structure 10 includes alightly doped p-type substrate 12 composed of semiconductor materialand, preferably, bulk Si material. In any of the embodiments, thesubstrate 12 may be composed of any suitable material including, but notlimited to, Si (e.g., single crystalline Si), SiGe, SiGeC, SiC, GaAs,InAs, InP, and other III/V or II/VI compound semiconductors. In furtherembodiments, the substrate 12 may be silicon on insulator technology(SOI) which includes an insulator layer on top of the semiconductorlayer and another semiconductor layer on top of the insulator layer. Theinsulator is formed by any suitable process such as separation byimplantation of oxygen (SIMOX), oxidation, deposition, and/or othersuitable process. The other semiconductor layer on top of the insulatorlayer can be fabricated using wafer bonding, and/or other suitablemethods.

Still referring to FIG. 1, a subcollector region 14 is formed within thesubstrate 12 by a doping process, e.g., ion implantation. Thesubcollector region 14 can be a highly doped n-type region, i.e.,phosphorus or arsenic, formed by ion implantation processes or otherknown diffusion processes such that no further explanation is requiredherein.

FIG. 1 further shows shallow trench isolation regions 16 formed withinthe substrate 12, isolating a collector region 24. The shallow trenchisolation regions 16 can be formed by conventional lithography, etchingand deposition methods known to those of skill in the art. For example,a resist formed over the substrate 12 is exposed to energy (light) toform a pattern (opening). An etching process with a selective chemistry,e.g., reactive ion etching (RIE), will be used to form one or moretrenches in the substrate 12 through the openings of the resist. Theresist can then be removed by a conventional oxygen ashing process orother known stripants. Following the resist removal, insulator material(e.g., oxide material) can be deposited by any conventional depositionprocesses, e.g., chemical vapor deposition (CVD) processes. Any residualinsulator material on the surface of the substrate 12 can be removed byconventional chemical mechanical polishing (CMP) processes.

Additionally, FIG. 1 shows diffusion regions 18 between the shallowtrench isolation regions 16. The diffusion regions 18 can be formed byion implantation or doping processes as described herein. The diffusionregions 18 will extend and electrically connect to the subcollectorregion 14. In embodiments, the diffusion regions 18 can be n-typedopants such as, e.g., As. The diffusion regions 18 can be referred toas reach through contacts.

In FIG. 2, a material 20 is formed over the substrate 12 and, morespecifically, an oxide or polysilicon material is formed over thecollector region 24, shallow trench isolation regions 16 and diffusionregions 18. In embodiments, the material 20 can be a high temperatureoxide (e.g., TEOS) deposited or grown using conventional processes suchthat no further explanation is required for a complete understanding ofthe present disclosure. In alternate embodiments, the material 20 is adeposited oxide material using a conventional CVD process. Inembodiments, the material 20 can have a thickness of about 100 Å to 2000Å; although other dimensions are also contemplated herein. An opening 22is formed in the material 20 to expose the collector region 24 (e.g.,composed of Si of the substrate material 12). The opening 22 is formedby conventional lithography and etching processes as already describedherein.

FIG. 3 shows an epitaxial material 26 deposited on the exposedsemiconductor material 12 in the collector region 24. In embodiments,the epitaxial material 26 above the collector region 24 will grow as asingle crystalline semiconductor material, e.g., Si, SiC or SiGe, usedto form part of the collector region 24 of the HBT. On the other hand,the epitaxial material 26 over the shallow trench isolations regions 16will grow into poly material, as should be understood by those of skillin the art. The epitaxial material 26 can also be deposited on thesurface of the material 20 which can be removed by a conventionalplanarization process, e.g., chemical mechanical polishing (CMP). Thefollowing description assumes that the epitaxial material 26 was eitherremoved or was only selectively grown on the exposed semiconductormaterial 12 in the collector region 24; although, the material 26 canremain on the material 20 for removal in subsequent processes.

FIG. 4 shows cavity structures 28 formed at a corner of the shallowtrench isolation regions 16 and within the collector region 24. Inembodiments, the cavity structures 28 extend into the epitaxial material26 and the substrate 12, at the corner of the shallow trench isolationregions 16 and also could extend into the poly Si region layer 20(adjacent to layer 26). Prior to forming of the cavity structures 28, apad film 30 of nitride material or oxide material or combination ofoxide and nitride is deposited onto the materials 20, 26 usingconventional deposition processes, e.g., CVD. The nitride material canbe deposited to a thickness of about 100 nm to 200 nm; whereas, theoxide material can be deposited to a thickness of about to 10 nm. Itshould be understood, though, that other thicknesses and combinations ofmaterials are also contemplated herein.

To form the cavity structures 28, openings or trenches 32 are etchedthough the pad film 30 and into the material 26, exposing the substrate12. In embodiments, the trenches 32 can extend into the substrate 12. Inembodiments, the trenches 32 can also be “holes” and/or “bars” (forminga continuous airgap structure). In embodiments, holes would have 1:1aspect ratio, while the bars would have aspect ratios >1:1. The trenches(holes and/or bars) 32 can be formed by conventional lithography andetching processes as already described herein such that no furtherexplanation is required for a complete understanding of the presentdisclosure.

A sidewall liner 23 is formed on the sidewalls of the trenches 32 bydepositing a dielectric material and anisotropic etching the dielectricmaterial from the bottom and top planar features of the structure. Thesidewall liner 23 should robustly coat the sidewalls of the trenches 32in order to protect the underlying material from subsequent etchingprocesses (for cavity formation). To achieve this robust sidewallcoverage, the dielectric material or materials needs to be thick enoughto leave a thick film on the sidewalls of the trenches 32 but not toothick that it pinches off the top opening of the trenches 32, whichwould prevent cavity formation during the successive cavity etch.

In embodiments, the sidewall liner 23 can be an oxide material, as anexample, composed of a combination of a thermal oxidization of thesubstrate 12 in a furnace to form to form a SiO₂ layer, followed by aCVD, atomic layer deposition (ALD), or any other known oxide depositionmethod. In embodiments, the anisotropic etch could consist of a RIEusing a perfluorocarbon-based chemistry, which etches material fromplanar surfaces but leaves dielectric material on the sidewall of thetrenches 32.

Prior to the cavity structure formation, an optional vapor or liquid HFtreatment, hydrogen plasma, anneal, basic or acidic chemical clean, orany process known to remove thin or native dielectrics or residualspacer etch polymer can be used to remove excessive dielectric at abottom of the trenches 32 (e.g., from the exposed substrate 12 (e.g.,silicon)). The post sidewall liner etch cleans (e.g., anisotropic etch)should still leave a robust dielectric sidewall liner 23 on the topcorner and sidewall of the trenches 32 to prevent etching of thesubstrate 12 or material 26 through the sidewall of the trenches 32during cavity formation.

As further shown in FIG. 4, the cavity structures 28 are selectivelyformed in the substrate 12 at a corner of the shallow trench isolationregions 16. The cavity structures 28 are formed by an etching processthrough the bottom of the trenches 32. During the etching process, thepad film 30 and the spacer films (e.g., sidewall liner) 23 protect thesubstrate 12 and the material 26 from being unintentionally etchedduring the cavity formation. In embodiments, the exposed substratematerial 12 and, in embodiments, portions of the material 26, can beremoved by a wet etching process or dry etching process. For example,dry etchants can include plasma-based CF₄, plasma-based SF₆, or gas XeF₄silicon etch, etc., and wet etching processes can include KOH and NH₄OH.To avoid unintentional etching of the substrate 12 or the epitaxialmaterial 26 on the sidewall of the trenches and top surface of thestructure, the pad film 30 and sidewall liner 23 completely cover thesubstrate 12 and the epitaxial material 26.

In FIG. 5, the sidewall liner and pad film are removed, exposing theupper surface of the materials 20, 26 and the sidewalls of the trenches32. In embodiments, the sidewall liner and pad film can be removed by aconventional etching process selective to such materials. For example,the sidewall liner and pad film can be removed by using only acombination of hot phosphorous followed by an HF chemistry or vice-versadepending on the single dielectric layer or stack of differentdielectric layers used for sidewall liner.

Following the removal of the sidewall liner and pad film, the trenches32 are subjected to an optional annealing process to soften or round(curve) the edges of the trenches. By way of one example, following anHF preclean process, the structure can undergo an annealing process at atemperature range of about 800° C. to about 1100° C., for up to about 60seconds. In more specific embodiments, the annealing process can be at atemperature of about 650° C. for 60 seconds performed in an H₂atmosphere; although other hydrogen atmospheres are also contemplatedherein, e.g., NH₃, B₂H₆, Phi, AsH₂ or other gases bonded to hydrogen. Inembodiments, the annealing in a H₂ or other hydrogen atmosphere mayremove any native or other oxide from the silicon based surfaces andwill smooth or reflow the walls of the cavity structures 28. If littleor no curvature is used, then the annealing temperature, time, orhydrogen-based gas flow is reduced to eliminate or minimize the siliconsubstrate reflow.

Still referring to FIG. 5, an epitaxial material 34 is deposited (e.g.,grown) on the material 20 and the epitaxial material 26. In embodiments,the material 34 is composed of SiGe used as the intrinsic (and/orextrinsic) base of the HBT. The SiGe material 34 can be deposited usingultra high vacuum CVD (UHVCVD), e.g., at a temperature of about 600° C.to 850° C., sealing the cavity structures 28 and thereby forming sealedairgaps 35. In an example, the SiGe material 34 can be heated to equalto or greater than the reflow temperature so that the SiGe material 34fills in the top of trench 32. Since SiGe has a lower reflow temperaturethan Si, the SiGe material 34 can be reflowed into the opening of thetrench 32 to plug or fill the top of the trench 32 without filling inthe cavity structure 28, itself, and hence forming a sealed airgap 35.In embodiments, the reflow temperature is 800-1050° C. and the reflowtime is anywhere up to about 600 seconds. In more specific embodiments,the reflow temperature is 850° C. and the reflow time is 60 seconds.

In this way, the sealed airgaps 35 are composed of the collectormaterial, shallow trench isolation material and sealed with SiGe basematerial (i.e., that is, the sealed airgap 35 has a triple interface,i.e., the interface of the extrinsic base, corner of the shallow trenchisolation and the collector region). Also, the sealed airgap 35 extendsvertically through part of the collector region 24. Moreover, the sealedairgap 35 can extend vertically through the intrinsic base, e.g.,material 34. As in any of the embodiments, the sealed airgap 35 can havea diameter of 0.1 μm; although other dimensions are contemplated herein.

FIG. 5 further shows additional intrinsic base material 36 deposited onthe material 34 (once the cavity structure 28 is sealed). Inembodiments, the additional base material 36 is doped SiGe material usedas the intrinsic base. The dopants can be, e.g., C or B. An emittermaterial 38 is deposited on the additional base material 36. The emittermaterial 38 can be epitaxial grown Si or SiGe material, for example.

FIG. 6 shows an emitter region 40 and extrinsic base region 42, amongstother features, and respective fabrication processes in accordance withaspects of the present disclosure. More specifically, theextrinsic/intrinsic base material 38 is subject to a doping process,e.g., ion implantation process, to form the extrinsic base region 42. Inembodiments, the ion implantation process uses a p-type dopant to form ahighly doped material as should be understood by those of skill in theart such that no further explanation is required for a completeunderstanding of the present disclosure.

Still referring to FIG. 6, the emitter region 40 is formed above thecollector region 24, in direct contact with the intrinsic base (e.g.,extrinsic/intrinsic base material 38). The emitter region 40 is composedof dielectric material and polysilicon material 46, with sidewalls 44.The sidewalls 44 can be a single sidewall material or multiple sidewallmaterials, e.g., oxide and/or nitrogen. The emitter region 40 is formedusing conventional lithography, etching and deposition processes suchthat no further explanation is required herein for a completeunderstanding of the present disclosure.

In FIG. 7, the materials 20, 34, 36, 38 are patterned using conventionallithography and etching processes to expose the subcollector region,e.g., diffusion regions 18 of the substrate 12, which electricallycontacts to the subcollector region 14 and collector region 24. Prior tocontact formation, silicide contacts 50 are formed in contact with thecollector region 24, e.g., subcollector region 14, emitter region 40 andthe extrinsic base 42. As should be understood by those of skill in theart, the silicide process begins with deposition of a thin transitionmetal layer, e.g., nickel, cobalt or titanium, over fully formed andpatterned semiconductor materials. After deposition of the material, thestructure is heated allowing the transition metal to react with exposedsilicon (or other semiconductor material as described herein) forming alow-resistance transition metal silicide contacts 32. Following thereaction, any remaining transition metal is removed by chemical etching,leaving silicide contacts 32.

A dielectric material 52 is deposited over the structure, e.g.,subcollector region 14, emitter region 40 and the extrinsic base region42, followed by a lithography, etching and deposition processes (e.g.,metallization process). For example, the dielectric material 52 isdeposited by a CVD process, followed by the lithography and etching(e.g., RIE) process to form trenches with in the dielectric material 52.A metal material, e.g., aluminum or tungsten, is deposited within thetrenches to form the contacts, e.g., collector contact 54 a, emitterregion contact 54 b and extrinsic base contact 54 c.

FIG. 8 shows a heterojunction bipolar transistor with an airgap sealedby emitter material in accordance with alternative aspects of thepresent disclosure. More specifically, in the heterojunction bipolartransistor 10 a of FIG. 8, the trenches 32 are formed through the layersof material 26, 34 and 36. The trenches 32 are formed in a similarmanner as described with respect to FIG. 4, with the addition of etchingthrough the materials 34, 36 using selective chemistries for suchmaterials. The cavity structures 28 are also formed within the substrate12, in addition to the collector region material 26. The trenches 32 arethen sealed with the epitaxial material 38 of the emitter material byconventional deposition processes as described herein. As in theprevious embodiment, the sealed airgap 35 extends vertically throughpart of the collector region 24 and also extends vertically through theintrinsic base, e.g., material 34, 36. The remaining structure issimilar to that described with respect to FIG. 7.

FIG. 9 shows a heterojunction bipolar transistor with an airgap sealedby epitaxial material and surrounded by collector material in accordancewith alternative aspects of the present disclosure. More specifically,in the heterojunction bipolar transistor 10 b of FIG. 9, the cavitystructures 28 are formed entirely within the substrate 12, i.e.,collector region 24, prior to the formation of the shallow trenchisolation regions 16.

In this embodiment, a trench and cavity are formed in the substrate 12,within the collector region 24. An epitaxial growth of material, i.e.,SiGe, is used to seal the cavity structure to form the sealed airgap 35.The epitaxial growth of SiGe material can be provided by the processesdescribed herein. Accordingly, in this embodiment, the airgap 35 issurrounded by collector material, i.e., Si, on all sides except for SiGeseal. After the SiGe grown, additional silicon can be grown, e.g., 0.8μm, followed by the formation of the shallow trench isolation regions 16and diffusions 18. As in any of the embodiments, an optional deep trench58 can be provided to isolate the subcollector region 14. The remainingstructure (and respective processes) is similar to that described withrespect to FIG. 7.

FIG. 10 shows a layout of trenches 32 used for airgap formation inaccordance with aspects of the present disclosure. In embodiments, thetrenches 32 are formed along a length of the collector region 24, at acorner of shallow trench isolation regions 16. In embodiments, thetrenches 32 can be in parallel rows along the emitter region with thedistance between the trenches 32 adjusted so that the “undercut” of thesealed airgap merges together to form a single (continuous) sealedairgap for lower Ccb. The trenches can be representative of holes with adimension of about 0.1 μm²×0.1 μm²; although other dimensions arecontemplated herein. In alternative embodiments, the trenches 32 can berepresentative of a bar with a dimension of about 0.1 um×the size of theemitter length, as an example.

The transistors can be utilized in system on chip (SoC) technology. Itshould be understood by those of skill in the art that SoC is anintegrated circuit (also known as a “chip”) that integrates allcomponents of an electronic system on a single chip or substrate. As thecomponents are integrated on a single substrate, SoCs consume much lesspower and take up much less area than multi-chip designs with equivalentfunctionality. Because of this, SoCs are becoming the dominant force inthe mobile computing (such as in Smartphones) and edge computingmarkets. SoC is also commonly used in embedded systems and the Internetof Things.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

1. A structure comprising: a subcollector region in a substrate; acollector region above the substrate; a sealed airgap formed at leastpartly in the collector region, the sealed airgap being sealed bysemiconductor material; a base region adjacent to the collector region;and an emitter region adjacent to the base region.
 2. The structure ofclaim 1, wherein a base material of the base region seals the sealedairgap.
 3. The structure of claim 2, wherein the base material is SiGe.4. The structure of claim 2, wherein the sealed airgap includes a trenchextending into collector material of the collector region composed ofcrystalline Si and poly Si, and which is sealed by the base material. 5.The structure of claim 1, wherein emitter material of the emitter regionseals the sealed airgap.
 6. The structure of claim 5, wherein theemitter material is SiGe.
 7. The structure of claim 5, wherein thesealed airgap includes a trench which extends into collector material,base material and which is sealed by the emitter material.
 8. Thestructure of claim 1, wherein the sealed airgap extends verticallythrough part of the collector region at a corner of shallow isolationregions.
 9. The structure of claim 1, wherein the sealed airgap extendsvertically through an intrinsic base region and the collector region.10. The structure of claim 1, wherein the sealed airgap is continuousalong a length of emitter region.
 11. The structure of claim 1, whereinthe sealed airgap includes isolated holes.
 12. The structure of claim 1,wherein the sealed airgap is surrounded by collector material of thecollector region and sealed by SiGe.
 13. A structure comprising: abipolar device comprising an emitter, base, collector and subcollector;shallow trench isolation regions isolating the collector; and at leastone sealed airgap at least partly within the collector and at a cornerof the shallow trench isolation regions, wherein the at least one sealedairgap include trenches which are sealed by either emitter material orintrinsic base material.
 14. (canceled)
 15. The structure of claim 13,wherein the at least one sealed airgap is provided within the collectorand is surrounded by collector material and sealed with epitaxialmaterial.
 16. The structure of claim 15, wherein the epitaxial materialis SiGe.
 17. The structure of claim 13, wherein the at least one sealedairgap is a plurality of sealed airgaps running parallel along theemitter.
 18. The structure of claim 13, wherein the airgaps are sealedwith epitaxial material.
 19. The structure of claim 18, wherein theepitaxial material is reflowed SiGe.
 20. A method comprising: forming asubcollector region in a substrate; forming a collector region above thesubstrate; forming a sealed airgap formed at least partly in thecollector region by sealing a trench extending in the collector regionwith semiconductor material; forming a base region adjacent to thecollector region; and forming an emitter region adjacent to the baseregion.
 21. The structure of claim 1, wherein the sealed airgap includetrenches which are sealed by either emitter material or intrinsic basematerial